Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for dampening a torsional vibration that appears in a rotating system.
The oil and gas industry has a growing demand for driving various machines at variable speeds. Such machines may include compressors, electrical motors, expanders, gas turbines, pumps, etc. Variable frequency electrical drives increase energy efficiency and provide an increased flexibility for the machines. One way for driving, for example, a large gas compression train is the load commutated inverter (LCI). However, a problem introduced by power electronics driven systems is the generation of ripple components in the torque of the machine due to electrical harmonics. The ripple component of the torque may interact with the mechanical system at torsional natural frequencies of the drive train, which is undesirable.
A torsional oscillation or vibration is an oscillatory angular motion that may appear in a rotor having various masses attached to it as shown for example in FIG. 1. FIG. 1 shows a system 10 having a gas turbine 12, a motor 14, a first compressor 16 and a second compressor 18. The shafts of these machines are either connected to each other or a single shaft is shared by these machines. Because of the impellers and other masses distributed along shaft 20, a rotation of the shaft 20 may be affected by torsional oscillations produced by the rotation with different speeds of the masses (impellers for example) attached to the shaft.
As discussed above, the torsional vibrations are typically introduced by the power electronics that drive, for example, the motor. FIG. 1 shows a power grid source (power source) 22 providing electrical power to the LCI 24, which in turn drives the rotor 20 of the motor 14. The power grid may be an isolated power generator. In order to damp (minimize) the torsional vibrations, as shown in FIG. 2 (which corresponds to FIG. 1 of U.S. Pat. No. 7,173,399, assigned to the same assignee as this application, the entire disclosure of which is incorporated here by reference), an inverter controller 26 may be provided to an inverter 28 of the LCI 24 and may be configured to introduce an inverter delay angle change (Δβ) for modulating an amount of active power transferred from inverter 28 to motor 14. It is noted that the term “delay angle” is considered in the literature as being synonymous with the term “firing angle.” For this reason, these terms are considered in this description to mean the same thing and may be used interchangeably. This assumption is true irrespective if alpha or beta modulation is considered. Alternatively, a rectifier controller 30 may be provided to a rectifier 32 and may be configured to introduce a rectifier delay angle change (Δα) for modulating the amount of active power transferred from the generator 22 to the DC-link and thus to the motor 14. It is noted that by modulating the amount of active power transferred from the generator to the motor it is possible to damp the torsional vibrations.
The two controllers 26 and 30 receive as input, signals from sensors 36 and 38, respectively, and these signals are indicative of the torque experienced by the motor 14 and/or the generator 22. In other words, the inverter controller 26 processes the torque value sensed by sensor 36 for generating the inverter delay angle change (Δα) while the rectifier controller 30 processes the torque value sensed by the sensor 38 for generating the rectifier delay angle change (Δα). The inverter controller 26 and the rectifier controller 30 are independent from each other and these controllers may be implemented together or alone in a given machine, dependent on the operation and the system sensitivity of the connected drive trains. FIG. 2 shows that sensor 36 monitors a part (section) 40 of the shaft of the motor 14 and sensor 38 monitors a shaft 42 of the power generator 22. FIG. 2 also shows a DC link 44 between the rectifier 32 and the inverter 28.
However, measuring mechanical properties, e.g., mechanical torque applied to a rotational shaft, can be expensive or impractical for high power drive trains. Sometimes, measuring the mechanical torque is not possible as the shaft is not accessible, or the shaft is surrounded by an explosive atmosphere, such as in gas compression applications. Accordingly, it would be desirable to provide systems and methods that avoid measuring the mechanical torque applied to the rotational shaft.